PROJECT SUMMARY Advances in cancer diagnosis and therapeutics over the last three decades have greatly reduced cancer mortality rates. However, this benefit was accompanied by a paradoxical rise of treatment-related cardiotoxicity, which has become the most common non-cancer cause of death among the 15.5 million cancer survivors in the United States. The most severe form of cardiotoxicity is Type I cardiotoxicity, which is predominantly caused by the anthracycline family of anticancer agents including doxorubicin (DOX, trade name: Adriamycin) due to induction of myocyte death. Younger age at treatment is a major risk factor for developing anthracycline cardiotoxicity. Most recent findings suggested that chemosensitivity in young tissues was caused by higher levels of apoptosis in response to cytotoxic chemotherapies. A key feature distinguishing children from adults is that the heart is still growing with active DNA synthesis and significant cell cycle activity. We hypothesize that elevated cell cycle activity in cardiomyocytes enhances cardiac chemosensitivity by accelerating apoptosis following DOX exposure. In support of this hypothesis, we have shown that expression of p21, a Cip/Kip family cyclin-dependent kinase (CDK) inhibitor that arrests the cell cycle, protected against DOX-induced apoptotic death of cardiomyocytes. By contrast, activation of CDK2 exacerbated DOX-induced cardiomyocyte apoptosis and promoted expression of the pro-apoptotic protein Bim. Importantly, DOX treatment induced CDK2 activation in vitro and in vivo, suggesting that CDK2 activation may represent a key mechanism underlying DOX-induced myocyte apoptosis and cardiotoxicity. Based on these findings, we propose to further tackle the role of cell cycle machinery in cardiomyocyte apoptosis and cardiac chemosensitivity by pursuing the following three Specific Aims: 1) Define the mechanisms of CDK2-mediated DOX cardiotoxicity; 2) Investigate the regulation of CDK2 activity in cardiomyocytes; 3) Identify the common regulator for cell cycle activity and cardiac chemosensitivity. The proposed studies will have the potential to uncover novel mechanisms underlying cardiac chemosensitivity, and may lay the foundation for developing new treatment strategies against anthracycline cardiomyopathy.